L06 - Taste & Smell

Question 1

What are ageusia and anosmia?

Answer 1

• Ageusia is loss of taste

- Anosmia is loss of smell

Question 2

What are the submodalities of taste and where on the tongue are these submodalities detected?

Answer 2

1 - Sweet

2 - Salt

3 - Sour

4 - Bitter

5 - Umami

• Although each taste cell has one type of chemoreceptor molecule to respond to one submodality, the different taste cells are not separated across the tongue (the tongue is not organised into discrete areas for different submodalities)

Question 3

Describe the structural organisation of taste buds.

Answer 3

• A pore exists on the surface of the taste bud
• This leads into a taste cell (not a neurone, but associated with a primary sensory afferent)
• The taste cell sits above a basal cell (basal cells replenish taste cells)
• Primary sensory afferents lead from the taste bud to the CNS

Question 4

Describe the generic transduction pathway for taste reception.

Answer 4

• A substance binds to the sensory receptor
• This induces a receptor potential (transduction)
• The receptor potential causes voltage-gated Ca2+ channels to open, causing calcium influx
• This causes “neurotransmitter” to be released into the synapse with the sensory afferent neurone

Question 5

How does transduction occur for salt?

Answer 5

• Sodium enters the taste cells via non-gated channels

- This causes depolarisation

Question 6

How does transduction occur for sourness?

Answer 6

• H+ enters the taste cell via TRP channels
• H+ also blocks K+ channels
• These events cause depolarisation

Question 7

How does transduction occur for sweetness, umami and bitterness?

Answer 7

• Sensory stimuli for sweet, umami and bitter bind to different GPCRs
• The families of receptor subunits of these GPCRs include T1R and T2R
• Binding to these GPCRs causes depolarisation

Question 8

How many neurones are involved in the taste sensation pathway and what is their course?

Answer 8

• 3:
• The afferent sensory neurone travels from the tongue / palate / pharynx to the gustatory nucleus in the fourth ventricle of the medulla, where it synapses with the second order neurone
• The second order neurone travels to the ventral posteromedial nucleus of the thalamus, where it synapses with the third order neurone
• The third order neurone travels to the primary gustatory cortex

Question 9

What structures are involved in the secondary pathways of taste sensation?

Answer 9

• Medulla (swallowing, salivation)
• Hypothalamus (satiety, palatability)
• Also project to the primary gustatory cortex and give a more detailed view of taste

Question 10

How does the sensitivity of taste compare to the sensitivity of smell?

Answer 10

Smell is more sensitive than taste

Question 11

What are olfactory epithelia?

Answer 11

• Found in the roof of the nasal cavity, below the cribriform plate
• Olfactory cells are neurones, and are replaced every 4-8 weeks by basal cells
• The stimuli-detecting receptors lie on cilia

Question 12

How do olfactory cells differ from taste cells in the way that they respond to submodalities?

Answer 12

Each olfactory cell has only one type of receptor molecule, but each receptor molecule is able to bind a range of different odorants

Question 13

What is population coding?

Answer 13

• Odorant chemicals can bind to a range of receptors, and the coordinated firing of multiple neurones allows odorant detection
• Some receptors bind particular odorants better than others so certain cells fire many more APs in response to a specific chemical than others
• The brain integrates signals to identify the smell

Question 14

Describe the common process of transduction in smell receptors.

Answer 14

• Many different receptor types, all using GPCRs so have the same transduction mechanism
• Odorant binds receptor -> G-protein mediated intracellular cascade -> opening Na+/Ca2+ channel -> depolarisation -> receptor potential & AP if threshold met

Question 15

How many neurones are involved in the smell sensation pathway and what is their course?

Answer 15

• 2:
• Olfactory neurones synapse with second order neurones in the olfactory bulb
• All olfactory cells of the same submodality will synapse at the same glomerulus in the olfactory bulb -> signalling is concentrated into a particular area -> single output along olfactory nerve
• Second order neurones will either project to the olfactory cortex and surrounding
  temporal lobe areas for perception of smell
• Others will lead to the olfactory tubercle where they synapse and project to the VPM nucleus in the thalamus (for integration with taste) then up to the orbitofrontal cortex where smells are recognised
• Some pass from the primary olfactory cortex to the limbic system which is important for feelings associated with smells